The provision of an endoprosthesis that provides immediate and long term functional reliability continues to be one of the critical objectives in orthopedic research. The typically critical aspect has been the inability to provide a simple easily installed trouble free anchorage to the medullary canal and the ability to remove the prosthesis without damaging the bone. Many techniques, structures and operating principles have been proposed heretofore conveniently characterized as (1) a stem impacted into the medullary canal; (2) a stem fixed mechanically in situ by screws, pins, expansion mechanisms or the like; (3) a stem fixed by cement, (4) a porous stem gradually and eventually anchored by bone growth into the stem voids; and (5) stems equipped with one or more resilient anchorage expedients.
Each of these prior proposals is subject to shortcomings and disadvantages. Illustrative embodiments of certain of these proposals are in the following U.S. Pat. Nos., viz; Fischer 3,846,846; Rostoker 3,906,550; Kahn 3,938,198; Rybicki et al 4,011,602; Rosenberg 4,013,071; Bokros 4,038,703; Pifferi 4,051,559; Berner et al 4,124,026; Grundel et al 4,167,047; Grell et al 4,177,524; Koeneman 4,292,697; and Koeneman 4,314,381.
Each of these methods presents problems that can lead to failure of the arthroplasty. The problems are as follows:
In devices that are impacted into the bone canal or held in place by expansion bolts or bone screws bearing against surrounding bone, the actual surface of contact between the prosthesis and the bone may be very small and areas of stress concentration will frequently occur. Clinical observations report loosening of the implant due principally to bone resorption.
Devices requiring the use of cement for anchorage are subject to problems associated with the toxicity of the cement, necrosis of the adjacent bone, incomplete filling of the desired intramedullary space, and the absence of resiliency. In addition, reports of long-term results of cemented total hip replacements with a minimum follow-up of ten years indicate that the loosing rate on the femoral side ranges between 30 to 50 percent.
Stem emplacements relying on bony ingrowth for attachment of the stem to the bone canal wall require excellent apposition of the stem. Typically a gap of one millimeter will exceed the growth capacity of the bone. In addition, if motion prior to fixation should occur, fibrous tissue results rather than new bone growth.
Prior proposals for resilient anchorages involve various expedients utilizing coil springs having portions of their convolutions in contact with the bone or strips of resilient material mounted on a supporting prosthesis stem and providing resilient backing for rigid elements in contact with the bone cavity. All have limited contact with the bone and lack provision for uniformly distributed equalized pressure in a controlled predetermined amount to substantially all portions of the bone cavity opposite the body of the prosthesis.